SCID patients with later onset were identified only in the last decade of observation. A positive relationship between the age at onset and the DD was found. Patients were subsequently classified based on the genetic alteration, pathogenic mechanism and immunological classification. One hundred eleven patients, diagnosed as typical or atypical SCID according to the European Society for Immune Deficiencies criteria, were included. To this purpose, we conducted a 30-year retro-prospective multicenter study within the Italian Primary Immunodeficiency Network. Aim of this study is to define presenting signs and natural history of SCID in a large cohort of patients, prior to hematopoietic stem cell or gene therapies. Due to the variability of clinical phenotypes, the diagnostic complexity and the heterogeneity of the genetic basis, they are often difficult to recognize, leading to a significant diagnostic delay (DD). Severe combined immunodeficiencies (SCIDs) are a group of inborn errors of the immune system, usually associated with severe or life-threatening infections. 16Department of Pediatric Hematology and Oncology, Bambino Gesù Children's Hospital, Rome, Italy.Nocivelli Institute for Molecular Medicine, Department of Molecular and Translational Medicine, University of Brescia, and ASST Spedali Civili, Brescia, Italy Nocivelli, University of Brescia, Brescia, Italy

14Department of Clinical and Experimental Sciences, Pediatrics Clinic and Institute for Molecular Medicine A.13Department of Advanced Biomedical Sciences, Federico II University, Naples, Italy.12Pediatric Immunohematology and Bone Marrow Transplantation Unit, San Raffaele Telethon Institute for Gene Therapy (SR-TIGET), IRCCS San Raffaele Scientific Institute, Milan, Italy.11Department of Child's and Woman's Health, Pediatric Oncology and Hematology, University of Padova, Padova, Italy.10Division of Pediatric Hematology and Oncology, Ospedale G.9Section of Pediatrics Immunology and Rheumatology, Department of Pediatrics, University of Pisa, Pisa, Italy.8Pediatric Hematology Oncology, Institute for Maternal and Child Health IRCCS “Burlo Garofolo”, Trieste, Italy.Orsola-Malpighi Hospital, University of Bologna, Bologna, Italy 6Paediatric Hematology Oncology Unit, Policlinico-Giovanni XXII Hospital, University of Bari, Bari, Italy.5Department of Public Health and Pediatrics, Regina Margherita Children Hospital, University of Turin, Turin, Italy.4Pediatric Immunology Unit, Anna Meyer Hospital, University of Florence, Florence, Italy.3Unit of Immune and Infectious Disease, University Department of Pediatrics DPUO, Children's Hospital Bambino Gesù, Rome, Italy.2Department of System of Medicine University of Rome Tor Vergata, Rome, Italy.

1Pediatric Section, Department of Translational Medical Sciences, Federico II University, Naples, Italy.

Emilia Cirillo 1 Caterina Cancrini 2,3 Chiara Azzari 4 Silvana Martino 5 Baldassarre Martire 6 Andrea Pession 7 Alberto Tommasini 8 Samuele Naviglio 8 Andrea Finocchi 2,3 Rita Consolini 9 Paolo Pierani 10 Irene D'Alba 10 Maria Caterina Putti 11 Antonio Marzollo 11 Giuliana Giardino 1 Rosaria Prencipe 1 Federica Esposito 1 Fiorentino Grasso 1 Alessia Scarselli 2,3 Gigliola Di Matteo 2,3 Enrico Attardi 2 Silvia Ricci 4 Davide Montin 5 Fernando Specchia 7 Federica Barzaghi 12 Maria Pia Cicalese 12 Giuseppe Quaremba 13 Vassilios Lougaris 14 Silvia Giliani 15 Franco Locatelli 16 Paolo Rossi 2,3 Alessandro Aiuti 12 Raffaele Badolato 14 Alessandro Plebani 14 Claudio Pignata 1 *

What mutation causes SCIDS